1. Field of the Invention
This invention relates to a throttle device for an internal-combustion engine and, more particularly, to an electronically controlled throttle device which controls the opening and closing operation of a throttle valve by driving an electric actuator according to a control signal.
2. Description of Related Art
An electronically controlled throttle device which controls an engine throttle valve by driving an electric actuator (e.g., a dc motor and a stepping motor) has been in actual use. The electronically controlled throttle device is used to control the amount of opening of the throttle valve to the optimum throttle opening for engine operating condition in accordance with an accelerator pedal opening signal and a traction control signal. In the throttle body, therefore, a sensor which is a so-called throttle sensor for detecting a throttle valve opening (throttle position) is mounted.
The throttle sensor generally adopted is a potentiometer type, in which a brush mounted on a rotor rotating together with a throttle valve shaft slides on a resistor provided on a substrate, thereby to output a potentiometer signal (sensor detection signal) corresponding to the throttle valve opening.
The throttle body is equipped with an electric actuator and a reduction gear mechanism for power transmission, and recently is further provided with a default opening setting mechanism for holding a wider initial opening (the default opening) of the throttle valve than the full-close position when the ignition switch is in off position (in other words, when no current is being supplied to the electric actuator).
Here, the full-closed position of the throttle valve is defined as a mechanically full-closed position and an electrically full-closed position. The mechanically full-closed position is the minimum opening position of the throttle valve defined by a stopper. The minimum opening is set at a position where the intake air passage is slightly opened from a full-closed position to thereby prevent the throttle valve from galling. The electrically full-closed position is the minimum opening position within the range of opening used in control, and is set, by the control of the electric actuator, at a position of a slightly wider opening than the mechanically full-closed position (e.g., about 1 deg. larger than the mechanically full-closed position).
The default opening (i.e., the initial opening when the ignition switch is in off position) is set to the amount of opening of the throttle valve which is opened wider than the above-described full-closed position (the mechanically full-closed position and the electrically full-closed position) (e.g., 4 to 13 deg. wider than the mechanically full-closed position). The default opening is set from the reasons: one for achieving the air flow rate necessary for fuel combustion for operation to be performed prior to engine warm-up at the time of engine starting (cold starting) without providing an auxiliary air passage (an air passage bypassing the throttle valve). During idling, the throttle valve is controlled towards decreasing the amount of opening from the default opening as the engine warm-up proceeds (in this case, the electrically full-closed position is the lower limit position). For another reason, the default opening is adopted to meet requirements for insuring self-running (limping home) in the event of a throttle control system trouble or insuring an intake air flow rate necessary for preventing an engine stall, and for preventing the throttle valve from being stuck with a viscous substance, ice, or other, on the inside wall of the throttle body.
As examples of the electronically controlled throttle device, known prior art has been stated in, for example, Japanese Laid-Open No. Sho 63-150449 Patent Publication, U.S. Pat. No. 4,947,815 specification, Japanese Translation of PCT Application No. Hei 2-500677 corresponding to the US patent, Japanese Laid Open No. Sho 62-82238 Patent Publication and its corresponding U.S. Pat. No. 4,735,179 specification, Japanese Laid-Open No. Hei 10-89096 Patent Publication, and Japanese Laid Open No. Hei 10-131771 Patent Publication.
The electronically controlled throttle device can control more accurately the air flow rate suitable for the operation of the internal-combustion engine than the mechanical throttle device which transmits the amount of depression of the accelerator pedal to the throttle valve shaft through an accelerator cable. The component count is increased because of the provision of an electric actuator, a default opening setting mechanism, and a throttle sensor. Therefore, downsizing, weight reduction and simplification of the throttle body, and further improvements in operation accuracy are demanded.